1) I bought mine from RS Components, but I see that Mouser and DigiKey sell them too (very brief search online for the only extant part, ML1A-11JW). The only other part was ML1B-11JW, the "robust" type that should be less prone to binding, but those went EOL a little while ago.
2) Signature Plastics may be able to get them produced as a group buy (if they still have tooling for that keycap mount and a suitable keycap profile), and Devlin advertise an ML keycap series (called ML series funnily enough —
http://www.devlin.co.uk/products/components/keycaps.html) but I don't know what their MOQ is, blank or printed.
3) They're a fully distinct design and require a custom PCB. Note that they contain a built-in jumper (the J in the part number) to help with PCB routing.
The alternative if you like small switches is to keep your eye out for
Kailh PG1350 series as those promise to be an improved ML, with linear and clicky options. The click bar design they use is also in the new SMD PG1511 series variants which can be bought now, but I've not seen PG1350 for sale yet. Note that these need their own custom keycaps. As do Alps SKFSAA (the nearest Alps switch to ML), which are available for sale NOS — they're not regular Alps mount.